1. Field of the Invention
The present invention relates to a copier, printer, facsimile apparatus, multifunction machine or similar electrophotographic image forming apparatus. More particularly, the present invention relates to a developing device using a two-ingredient type developer made up of toner particles and carrier particles, a process cartridge and an image forming apparatus including the same.
2. Description of the Background Art
It is a common practice with a color copier, color printer or similar image forming apparatus to execute development with a two-ingredient type developer consisting of toner particles and carrier particles, as taught in, e.g., Japanese patent laid-open publication No. 2004-212560. The developer of the type mentioned may or may not contain additives coating the surfaces of the particles. It is generally accepted that a developing system using the two-ingredient type developer is advantageous over a developing system using a single-ingredient type developer, i.e., toner in that it makes the chargeability of toner stable for thereby implementing stable, high quality images.
Japanese patent laid-open publication No. 2004-212560 mentioned above, for example, pertains to an image forming apparatus of the type including a developing system using a two-ingredient type developer, which contains a carrier having a small particle diameter, and using only a DC bias for development. The image forming apparatus is configured to reduce carrier deposition and control granularity, local omission around characters and other defects of images. For this purpose, the static resistance and saturation magnetization of the carrier are optimized when use is made of carrier particles having a diameter as small as 20 μm to 60 μm.
Japanese patent laid-open publication No. 7-140723, for example, proposes to use a carrier containing carbon black as a resistance control agent for the purpose of obviating carrier deposition and other defects and stabilizing the amount of charge against aging.
A problem with conventional technologies is that cumulative time consumed to agitate a developer increases due to repeated development or aging, making it impractical to surely obviate granular images. This is particularly true when use is made of a two-ingredient type developer in which a carrier contains carbon black as a resistance control agent.
In the image forming apparatus disclosed in laid-open publication No. 2004-212560 mentioned previously, particular conditions, including static resistance and saturation magnetization, are assigned to the carrier having a small particle diameter in order to reduce carrier deposition as well as granular images and local omission around characters. However, the developer stored in a developing device is subject to stress ascribable to, e.g., agitation over a long time due to repeated development, so that toner also contained in the developer is damaged and lowers the fluidity of the entire developer. Consequently, it is likely that a granular image, controlled as expected in the initial stage, is produced due to aging. Granularity is particularly conspicuous when the carrier has a small particle diameter or when the amount of the developer is increased.
Laid-open publication No. 7-140723 also mentioned previously is expected to obviate carrier deposition and stabilize the amount of charge with carbon black contained in the carrier as a resistance control agent. However, it is likely that carbon black contained in the carrier is transferred to toner due to aging causes the toner to cohere, lowering the fluidity of the entire developer and therefore causing granularity to appear in images.
Technologies relating to the present invention are also disclosed in, e.g., Japanese patent laid-open publication No. 2000-089549.